1. Field of the Invention
The present invention relates to a multi charged particle beam writing apparatus, and more specifically, for example, relates to a method of writing a pattern on a target object on the stage by radiating beams.
2. Description of Related Art
The lithography technique that advances miniaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in the semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. The electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for writing or “drawing” a pattern on a wafer and the like with an electron beam.
As an example employing the electron beam writing technique, a writing apparatus using multiple beams (multi-beams) can be cited. Compared with the case of writing a pattern by using a single electron beam, since it is possible to emit multiple beams at a time in multiple writing, the throughput can be greatly increased. In the writing apparatus employing a multi-beam system, for example, multiple beams are formed by letting an electron beam emitted from an electron gun assembly pass through a mask with a plurality of holes, blanking control is performed for each of the beams, and each unblocked beam irradiates a desired position on a target object or “sample” (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2006-261342).
In a multi-beam writing apparatus, multiple beams are reduced and transmitted at a high reduction rate by using a reduction optical system composed of a group of lenses, such as a doublet lens. If the number of beams is large like the case of multi-beams, since the region to be reduced and transmitted is also large, it becomes necessary to suppress distortion aberration to be small in order to maintain writing precision. If distortion aberration is made small in the lens group of the reduction optical system, the working distance (WD) becomes short and there occurs a problem that the last lens and other devices using the space between the last lens and the target object surface interfere with each other. Of course, also in the single beam system, this problem will probably be serious as further writing precision is to be required.